The present invention relates to a method of allocating channels for a radio link in a radio link system.
Modern terrestrial microwave radio systems provide a feasible technical solution for telecommunications transmission links at distances from some hundreds of meters up to 80 km. Such systems are increasingly being developed in both cellular and fixed telecommunications networks. In the latter case, particularly in wireless based networks, and in the former case, in base station interconnection and a base station-a base station controller connection, a radio link system is particularly in urban areas a good solution. Unlike fiber, which can require several months for right-of-way and permits, microwave can be put into immediate operation. In addition, microwave easily goes over difficult terrain where cable cannot be laid, and microwave does not require trenching or pulling through duct work, which can take weeks or months and which increases installation costs.
A typical microwave radio site consists of an indoor mounted base band unit, an indoor or outdoor mounted radio frequency transceiver, and an parabolic antenna.
Basically there are two types of radio link network topologies in use, namely star networks and ring networks. Of course, it is common for hybrid ring and star network to be deployed.
FIG. 1 depicts an example of the star network. It contains one or more hub sites at strategic locations which serve spurs or chains of subordinate sites from the centralized hub. The hub sites are connected to the switch via a transmission link which usually is a trunk cable. The star network has one disadvantage in that outages on a single transmission link may affect many sites so lowering overall network reliability.
FIG. 2 shows a network configured in a ring structure. This structure requires some routing and grooming intelligence at all appropriate points in the network. The capacity of every link in the ring has to be sufficient to support all sites in the loop.
As mentioned above, radio link network provides one solution for realizing a cellular telecommunications network. Then, with reference to FIGS. 1 and 2, the switch might be a mobile switching centre, hub site can be a base station controller and subordinate site is a base transceiver station. Each of the radio links performs a point-to-point connection.
A message, be it audio, video, or data is modulated on the microwave signal, which is often referred to as a carrier. The maximum distance between sites, also called a hop distance, is mainly determined by propagation characteristics of electromagnetic waves. The higher the carrier frequency the greater freespace loss, or attenuation due to the atmosphere, i.e. the shorter the achievable distances. However, this also means that frequency re-use distances are shorter: the distance between links operating on the same frequency can be shorter without fear of interference. There are three types of interference which should be considered in any terrestrial radio link network: 1) intrasystem occurs when a radio signal within a multi-hop network interferes with the receiver of a different hop. 2) external disturbance occurs when a foreign system affects a signal. 3) reflectionxe2x80x94from anything that has a reflective surface can deflect other signals into the path of the transmitted signal and the stronger signal will interfere with the weaker signal.
Radio links have traditionally operated on regulated frequency bands which further are divided to frequency channels. The use of radio channels is regulated by local authorities and based on coordinated planning. Hence, in a predetermined local area in which radio links are to be established, only a predetermined overall bandwidth and then a predetermined number of channels are available for the radio links.
When a plurality of radio links or so-called hops are present within a given area, in the regulated radio environment, the channel choice is based on coordinated frequency planning. That is, the channel to be used for a specific radio link at a time is predetermined. Nevertheless, in such a regulated radio environment, the channel to be used for a link may be changed. In other words, a channel allocation for a radio link may be periodically updated and changed.
In the planning, each radio link is represented as a variable whose domain is the set of all frequencies that are available. The objective is to assign frequencies to the radio links in order to avoid interference. Prior the planning, it is essential to determine, at the earliest opportunity, what band are locally available for fixed link systems, and what the local xe2x80x9clink policyxe2x80x9d is. The majority of national frequency management administrations have some form of link policy regarding link lengths and net output power expressed as an equivalent isotropically radiated power (EIPR).
Recent developments in telecommunications have, however, lead to changes with regard to frequency allocations and have thus created possibilities to operate radio links and/or hops in non-coordinated frequency bands. These specific bands are left unregulated in the sense that selection of a working channel for an individual radio terminal inside the band is not controlled by the local authorities. Instead, the channel can be selected freely as long as the general requirements associated with the band are not violated. As an example, European Telecommunication Standard ETS 300408 specifies the minimum performance parameters for radio equipment operating at frequencies around 58 GHz and not requiring coordinated frequency planning. Within this band it is of interest to share the bandwidth among different links in an efficient way.
However, this means that unlike the further above described traditional radio links within a regulated (or coordinated) radio environment, those systems operating in an non-coordinated band will operate in interference limited environment. That is, the signal quality of received signals may be deteriorated due to interference phenomena caused by neighboring radio links. Therefore, it is of increasing interest to consider how to share available bandwidth among various systems in an efficient way.
A state of the art approach for radio links operating in an non-coordinated band resides in assigning a fixed channel to each radio link or hop already at the stage of production of the respective devices at the factory. This is, for example, the approach adopted by the company xe2x80x9cMicrowave Modules Ltd.xe2x80x9d, which produces radio links for the non-coordinated 58 GHz band.
These devices which are used to establish point-to-point local networks are using fixed channel allocation principle. Various problems as explained below may arise during operation of the system.
FIG. 3 illustrates a simplified example for a prior art non-coordinated link system and the problems associated therewith. Let us assume that within the geographic area only three channels (channel numbers 1, 2, 3) for radio links (hops) are available. Now, transceivers in the sites 21 and 22 are configured at the factory side to transmit on channel 1 so that the hop between these sites is using channel 1. Transceivers in the sites 25 and 26 are configured to transmit on channel 2 and in the sites 23 and 24 channel 3 has been preset at the factory. Due to different channels, i.e. different frequencies, this three links operate well without disturbing each other.
But, if a fourth hop, sites 27 and 28, is to be additionally established within this area, some difficulties arise. The transmitters in these sites are configured at the factory side to transmit on a specific channel denoted by channel x, x being 1, 2 or 3 in the chosen example. Consequently, due to the arrangement or configuration of the hops and the respective fixed channels thereof, a channel collision between one of the xe2x80x9coldxe2x80x9d hops and the new hop is extremely likely to occur in the depicted situation, irrespective of which channel (channel 1, 2, or 3) has been chosen for transmitters in sites 27 and 28.
The term channel collision in this connection means crosstalk or interference phenomena which are likely to occur between respective hops and result in a decreased transmission quality. In particular, a channel collision is defined as occurring for a radio link for which a ratio of S/I is below a given collision threshold THc, i.e. S/I less than THc, with S representing signal power and I representing interference power from one or several other radio links within the same radio environment.
That is, with reference to the schematically depicted example of FIG. 3 there may either occur a channel collision between the new hop and the old hop using channel 1 in case the new hop transmits on channel 1, or a channel collision between the new hop and the old hop using channel in case the new hop transmits on channel 2, or a channel collision between the new hop and the old hop using channel 3 in case the new hop transmits on channel 3.
To be precise, for a given number of randomly placed hops within a radio environment of a well defined area, channel collisions between respective hops are very likely to occur. This, in turn, severely limits the number of radio links (hops) per area (km2) of the radio environment to a value much lower than a value which should desirably be achieved.
Moreover, fixing the operating frequency (channel) of each radio link terminal at the factory does not result in a globally optimal distribution of channels. It also adds extra task in the manufacturing process. Furthermore, it complicates the planning of the radio link frequency usage in a network.
It is therefore an objective of the present invention to generally provide a method of allocating channels for a fixed radio link operating in an non-coordinated frequency band, which method allows the number of links that can be commissioned in a given area to be increased by a large factor while simultaneously lowering the above described risks of the non-coordinated frequency band.
Preferably, allocation of the radio channel for each of the links should be taken place automatically.
According to the invention, for each radio link to be commissioned channels, interference level measurement on each available channel of the frequency band is carried out. It means that channel by channel, effect of each possible interfering source to the channel frequency being examined is automatically taken into account. Sources may be transmitters of the already commissioned radio links which are using the same or near the same frequency, sources from other radio systems etc. Based on the interference measurements, best transmission and reception channel for the link are chosen. Thereafter, the radio link can be commissioned and measurements in the next radio link to be commissioned can be started.
In accordance with one embodiment, for each radio link to be commissioned, the distance from each of the already commissioned radio links having an available channel allocated thereto is measured, and based on the distance measurement, the channel or channels to be allocated to the respective radio link to be commissioned are chosen.
Stated in other words, instead of using a fixed and pre-set channel for the fixed radio link, a channel is assigned autonomously and automatically when commissioning the link, depending on interference measurement results and/or distance measurement results. In a frequency band where the individual channels are freely selectable the possibility for automatic channel selection decreases the amount of commissioning work by removing the need for detailed frequency planning and fixed channel setting at the factory for each radio.
Another advantage is that the proposed method also ensures an efficient distribution of the channel usage in the network. If the channel allocation were based on fixed frequency settings, the resulting channel distribution in the network would not be optimal in terms of maximum hop density. In contrast, the automatic channel selection utilizes an algorithm where the channel selection is based on the measured interference from other radio links and makes it possible to install a larger number of hops in a given area.
Accordingly, due to the above described methods for allocating channels for a fixed radio link operating in a non-coordinated frequency band, the present invention provides the advantage that the number of links (hops) that can be-commissioned in a given radio environment area can be increased by a large factor. At the same time, the method effectively allows the risks of the non-coordinated frequency band to be significantly lowered.